Summary Abstract Immune regulation is critical to health and disease. Immune function is of clear relevance in the setting of hematopoietic cell transplantation and organ transplantation tolerance where controlled immune reactions allows for reduction of graft vs host disease (GVHD) risk and organ acceptance whereas rejection is manifested through GVHD and destruction of the organ graft. Prior work in this program in our laboratory has demonstrated that immune regulatory cells play a major role in control of these immune reactions allowing for the control of GVHD, retention of graft-versus-tumor effects, as well as acceptance of genetically disparate tissue grafts. These studies have been translated to the clinic in a number of different settings including the development of the total lymphoid irradiation and anti-thymocyte globulin (TLI/ATG) regimen pioneered in Project 1 for solid organ transplantation tolerance demonstrating the clear clinical relevance of these strategies to the treatment of patients. In this project, we will dissect the role of two important donor derived regulatory cell populations including CD4+CD25+FoxP3+ regulatory T cells (Treg) and CD1 reactive invariant natural killer T cells (iNK T cells). Prior work in my laboratory has demonstrated that both of these cell populations suppress GVHD across major histocompatibility and minor histocompatibility barriers and that importantly there are specific interactions between these different cell populations. A major challenge has been the paucity of these cell populations for clinical translation. In our proposal, we will build upon recent work using MAbs, fusion protein biologics and natural ligands to study their impact on Treg and iNKT cells and to develop strategies to activate and expand these cell populations. We will further explore important molecules involved in the mechanism of these regulatory cell populations, as well as their interaction with other host myeloid cell populations required for biological function. We will explore the role of these immune regulatory cells following both total body irradiation myeloablative as well as TLI/ATG conditioning to extend and build upon results from the other projects to develop novel insights into immune regulatory mechanisms of action and interactive networks and to develop clinically relevant strategies for translation. Project 2 will explore the underlying biology of these immune regulatory cells, as well as to develop strategies that can be translated to the clinic in current and future clinical trials. This project will interact with Projects 1 and 4 and extensively utilize Cores A and B.